Use the APInt versions of the bit-wise conversions of float/double to int

Use the APInt versions of the bit-wise conversions of float/double to int
and back. While this is not strictly necessary, it does pave the way for
future changes in casting. It should now be possible to improve vector
casting to deal with un-equal fp/int sizes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@34808 91177308-0d34-0410-b5e6-96231b3b80d8